In vivo metabolic imaging and monitoring of brown and beige fat.

Activation of the thermogenic brown and beige fat is an effective means to increasing whole-body energy expenditure. In this work, a unique label-free method was developed to quantitatively assess the metabolism and thermogenesis of mouse adipose tissues in vivo. Specifically, an optical redox ratio (ORR) based on the endogenous fluorescence of mitochondrial metabolic coenzymes (nicotinamide adenine dinucleotide and flavin adenine dinucleotide) was used to measure the metabolic state of adipocytes. Our findings demonstrate that the ORR provides a label-free and real-time biomarker to determine the thermogenic response of brown, beige and white adipose tissues to a variety of physiological stimulations. In addition, the redox ratio also can be used to evaluate the degree of browning in the white fat of cold-acclimated mice. This technique is important to understand the recruitment and activation of thermogenic adipocytes in mammals and thus can help to develop therapeutic strategies against obesity.